October 09, 2017
5 min read

The Asia Pacific CTO Algorithm: Think Global, Act Local

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The development of the hybrid algorithm to guide crossing of coronary chronic total occlusions was a pivotal event in chronic total occlusion PCI. Published 5 years ago, this algorithm has facilitated procedural planning and execution, as well as education and research. Together with rapid evolution in technology and techniques, the hybrid algorithm has led to high success with acceptable complication rates at multiple centers in North America and Europe.

The key components of the algorithm are:

  • careful review of the angiogram focusing on four parameters: proximal cap, lesion length, quality of distal vessel and presence of “interventional” collaterals that could be used for the retrograde approach;
  • use of these parameters to select the starting crossing technique; and
  • early change to another strategy if the initially selected one did not allow satisfactory progress.
Emmanouil S. Brilakis

The hybrid algorithm has been criticized, in large part because it does not include parallel wiring as one of the suggested strategies. Proponents of the parallel-wiring technique argue that in case of subintimal guidewire entry, advancing a second guidewire “parallel” to the first one will often succeed in crossing and carries significantly lower cost than use of dedicated dissection/re-entry devices. Similarly, the master hybrid algorithm does not include IVUS-guided wiring, although this is one of the options for the “sub-algorithm” on how to approach an ambiguous proximal cap. Moreover, the definition of “early conversion” has been controversial, as “too rapid” change could decrease the chances of the initially selected technique being successful. The hybrid proponents have counteracted that procedural efficiency is key for CTO PCI (or any PCI), and prolonged procedures are not only unpleasant for both patients and operators, but may also expose the patients to increased risk for coronary complications and radiation injury, among other things.

Dissecting the Asia Pacific Algorithm

The Asia Pacific CTO Club recently created the Asia Pacific algorithm for CTO PCI (see Figure). Initial results of the registry were presented at the 2017 Japan CTO Club in Nagoya, Japan, showing a 95% success rate. The algorithm is easily accessible on the Asia Pacific CTO Club website: http://apcto.club/apcto-algorithm/. I strongly believe that navigating the Asia Pacific algorithm can be a fruitful exercise that can provide valuable lessons for both sides of the Pacific.

Click here for larger image.

Source: Figure courtesy of Dr. Etsuo Tsuchikane; printed with permission

Similar to the hybrid algorithm, the first step of the Asia Pacific algorithm is careful analysis of the angiogram and also coronary CT angiography. One cannot overemphasize the critical importance of detailed angiographic review that should be done for at least 15 to 30 minutes. What we see after 30 minutes of angiographic review is often completely different (richer and more detailed) that what we see after 5 minutes. Coronary CT angiography is performed relatively infrequently in North America for guiding CTO PCI, but its use has been recently increasing, especially with the advent of CT-derived fractional flow reserve and is likely to increase more in the future with ongoing advances in CT/angiogram co-registration, which can provide real-time fusion imaging to guide CTO crossing. For in-stent restenosis lesions, upfront use of the CrossBoss Coronary CTO Crossing Catheter (Boston Scientific) is recommended in the Asia Pacific algorithm, a very reasonable strategy that has been shown to provide high success rates and often rapid crossing.


In contrast to the hybrid algorithm that recommends a primary retrograde strategy for ambiguous proximal cap, the Asia Pacific algorithm recommends IVUS-guided entry first. This is also quite reasonable, as the retrograde approach has fourfold higher risk compared with antegrade crossing. An alternative antegrade strategy not mentioned in the Asia Pacific algorithm, yet increasingly being used, is the “move the cap” dissection/re-entry group of techniques, in which a dissection plane is intentionally created proximal to the proximal cap, allowing subintimal CTO crossing with a knuckled guidewire.

Similar to the hybrid algorithm, if the distal vessel is of poor quality, a primary retrograde approach is recommended, provided there are appropriate “interventional” collaterals. Poor distal vessel quality poses a challenge for all antegrade strategies, both antegrade wire escalation (as subintimal entry is highly likely) and antegrade dissection/re-entry (it is more challenging to re-enter into small, diffusely diseased distal vessels).

If antegrade crossing is selected, both antegrade wire escalation, including parallel wiring and IVUS-guided wiring, and antegrade dissection/reentry with the CrossBoss/Stingray CTO system (Boston Scientific) are listed, but parallel wiring and IVUS-guided wiring is preferred in most cases in the Asia Pacific algorithm. This is probably the most prominent difference between the hybrid and Asia Pacific algorithm, and reflects local areas of strength. Expert Japanese operators have a long tradition of obtaining excellent outcomes with parallel wiring and the use of IVUS, having the patience, skill and equipment, such as the Gaia and Sion wires (both Asahi Intecc), to achieve crossing in many cases. In contrast, U.S. operators rely less on wire skills and more on the ability to re-enter into the distal true lumen if the guidewire enters the subintimal plane.

Change in Crossing

Change from antegrade to retrograde crossing and vice versa is included in the Asia Pacific algorithm, although it appears to be less emphasized than in the hybrid algorithm. If both antegrade and retrograde approaches fail, limited antegrade subintimal tracking (LAST) is listed as the next option. LAST refers to re-entry into the distal true lumen using guidewires. However, LAST is infrequently used currently because it may create a large hematoma precluding successful use of the Stingray system and/or potentially leading to very distal re-entry with high restenosis rates. If re-entry fails, it is often best to perform an “investment” procedure, ie, balloon the subintimal space but not implant stents and repeat angiography in 2 to 3 months, when a channel is often established through the occlusion, allowing rapid crossing and stenting.


When to Stop

Finally, the Asia Pacific algorithm ends by providing recommendations about when to stop: “If the procedure lasts > 3 hours, the volume of contrast administered is > 3.7 x the estimated [glomerular filtration rate], or the air kerma radiation dose is > 5 Gray (unless the procedure is well advanced).” These are very reasonable recommendations and are in line with the current practice of most CTO operators. Moreover, the Asia Pacific algorithm provides four sub-algorithms for various steps of the retrograde approach, which is consistent with the current trends of having a (simple) main algorithm and various sub-algorithms for tackling specific complex subsets, such as the ambiguous proximal cap, the impenetrable proximal cap, the balloon uncrossable CTO and the aorto-ostial CTOs.

Similarities, Differences

As is evident from the above discussion, the Asia Pacific and the hybrid CTO PCI algorithms have more things in common than not. They both emphasize the importance of careful angiographic review and build subsequent treatment strategies on the same anatomic criteria, ie, proximal cap morphology, quality of distal vessel and presence of interventional collaterals.

The differences in suggested strategies (antegrade dissection re-entry favored in hybrid vs. IVUS-guided wiring and parallel wiring favored in the Asia Pacific algorithm) reflect local practice patents, local strengths and traditions and different access to dedicated dissection/re-entry equipment.

‘Convergence in CTO PCI Practices’

Who is right? As is often the case, both sides are. Excellent outcomes can be achieved with both the hybrid and the Asia Pacific algorithm. Both algorithms are living examples of a convergence in CTO PCI practices around the globe that is likely to drive higher success and lower complication rates and facilitate training and future equipment development. As discussed in one of the prior CTO Corner columns in Cardiology Today’s Intervention, cross-pollination has been a key component of CTO PCI and will likely continue and expand into the future. Working together, we can all improve and offer the “best of both worlds” to our patients.

Disclosure: Brilakis reports receiving consultant and speaker honoraria from Abbott Vascular, Asahi, Cardinal Health, Elsevier, GE Healthcare and St. Jude Medical; research support from Boston Scientific and InfraRedx; and his spouse is an employee of Medtronic.